The general principle of such actuators is known and described for example in the international patent application WO 2010/027701. This patent application describes an actuator with a screw comprising two threaded parts with opposite threads engaged by motors turning in opposite directions in order to move the screw in translation with a greater force than with a system comprising only one motor. The opposite threads cancel out the resulting torque exerted on the screw by the two motors.
Another patent application WO 2013/027197 describes an electric linear actuator comprising an electric motor with a rotor and a stator, a housing and a nut-and-screw system for moving a member to be moved at least in translation in a direction A corresponding to the rotation axis of the rotor, the nut-and-screw system comprising a screw with a first threaded portion and a first nut with a complementary thread engaging with the first threaded portion, the first nut being coupled so as to rotate with the rotor. The screw comprises a second threaded portion and the nut-and-screw system comprises a second nut engaging with the second threaded portion, the second threaded portion and the second nut being configured so as to apply a reduction factor in the linear movement of the screw (with respect to linear movement due to the first threaded portion and to the first nut alone), the first and second threaded portions having opposite threads with respect to each other. In one embodiment, the first threaded portion is adjacent to the second threaded portion.
The European patent EP 1561639 describes an actuator intended for adjusting the horizontal or vertical position of car headlights that are of high performance even under extreme environmental conditions, in particular for temperature ranges from −40° C. to +120° C., and being resistant to dust, water and salt. The European patent EP 1791242 describes an actuator for the linear movement of a member to be controlled, comprising a housing, a motor and a nut-and-screw system. The motor, which is preferably a stepping motor, comprises a stator comprising coils and a rotor comprising permanent magnets, the rotor being supported, at one of its ends, by a bearing in the form of a ball bearing, comprising a race fixed to the housing and an output race secured to the rotor. The international patent application WO 03048599 describes a screw actuator comprising a nut element interacting with two separate coaxial screw threads and two screws that each have a different thread corresponding to the screw threads of the nut element.
In many applications, the actuators must comply with the working conditions of the automobile field, which imposes very wide temperature ranges, normally from −40° C. to +140° C., without significant loss of performance. The electric linear actuators of the prior art are, in the great majority of cases, constructed from a rotary motor coupled to a movement transformation in order in the end to give rise to a translation. The simplest solution is in fact using a rotor mounted so as to rotate on bearings or roller bearings, the said rotor driving a nut and screw system that generates the translation of the control member.
This solution has numerous advantages with regard to compactness and reliability in severe environments but it turns out that the performances of these actuators drop significantly as soon as they are used at very low temperature (between −20° C. and −40° C.), because of the variation in viscosity of the greases used in the ball bearings and bearing housings. This problem is complicated to solve since it is possible to find greases that remain fluid and lubricate correctly between +20° C. and +140° C. or other types that remain so from −40° C. to +25° C., but the only greases that make it possible to fulfil the function of lubrication (resistance to pressure without rupture of the film) without great variation in viscosity (which creates a brake on the bearing) over the whole of the range −40° C. to +140° C. are limited to rare compositions based on silicone. However, this silicone element is proscribed in the majority of automobile applications because of its effect of degassing and pollution of the peripheral elements.
The result is a real problem in offering automobile linear actuators working at −40° C. to +140° C., and offering the required lubrication for ensuring the service life of the product, without drops in performance when cold, due to the great viscosity of the grease that brakes the bearing significantly. At the present time the majority of linear actuators of the prior art are based on the combination of a rotary motor and a nut and screw system. These lose more than half their force when passing from 25° C. to −40° C. This leads to oversizing the motor in order to compensate for this loss and to achieve the specification expected by the user.